Electronic image quality can frequently be improved by making adjustments to image sharpness. Enhanced sharpness provides a crisper image with better object delineation and definition. In fact, enhanced sharpness is often desired because original scenes within an image can become degraded by optical losses and spatial sampling limits of the device that processes the image. Additionally, image processing applications with a digital computer, such as encoding, compressing, and the like can also cause detail within an image to be lost or distorted and can introduce noise. Furthermore, output devices (e.g., liquid crystal displays (LCDs)) can also corrupt image quality due to the bandwidth limitations of the electronic circuitry, physical cell structures and cross-talk between cells. The circuitry and cell structure may set an upper limit on the degree of spatial detail that can be reproduced within a given output device.
A human's visual capabilities can also affect one's perception of image quality. The human visual system is limited in its ability to resolve spatial detail in a scene because of the sampling characteristics of the retinal photoreceptors and the neural network of interconnections between them. Also, the human visual system has a built-in edge enhancement system that assists in the cognitive processes of image scene segmentation and interpretation. As a result, people prefer to see sharp images, which assist their interaction with, and understanding of, the natural world.
A conventional technique for image sharpening is referred to as unsharp mask (USM). USM generates an enhanced signal (fringe signal) from the difference between an original signal (sharp signal) at a single pixel and an average of signals (an average unsharp signal) that surround the pixel. A fraction of the fringe signal is then added to the sharp signal to produce an enhanced edge gradient. Although, decent improved sharpness can be achieved with USM, it is inflexible because it is limited to a single spatial frequency band within the image being processed and it takes no account of device or viewing characteristics.
Thus, there is a need for improved image sharpness management, which is not limited to a single spatial frequency band of an image being processed, and which allows compensation for device and human visual characteristics.